1. Field of the Invention
The present invention relates to an optical module, and more particularly, to an optical module using a matching circuit which includes a double stub or a double balanced open stub, a grounded waveguide, a microstrip line transition region and a microstrip line.
2. Discussion of Related Art
Recently, with the development of wireless communications, technologies for transmitting broadband signals to subscribers have been developed. A radio over fiber (ROF) technology for transmitting RF signals through a fiber has been popularly used as a method for transmitting various data from a base station to subscribers without loss.
Researches have been made on impedance matching of an optical modulator module, a laser diode module and an optical receiver module used for an ROF link for minimizing loss of RF signals by transmitting the signals through an optical fiber. Generally, a lumped type device has been used for impedance matching. However, since a maximum frequency is restricted by an RC time constant by a capacitance of the device and a load resistance, the device has difficulty in processing high speed electric signals of 40 GHz and 60 GHz. In addition, a bandwidth is fixed in a specific frequency and not efficiently controlled.
One example of the impedance matching structure of the lumped type optical device has been disclosed under U.S. Pat. No. 6,101,295. A conventional lumped type optical module will now be explained.
FIG. 1 is a schematic plan view illustrating an optical module having an impedance matching structure for the conventional lumped type optical device.
Referring to FIG. 1, a device 1 and a signal line 5 of a microstrip line of a dielectric substrate 3 on which an impedance matching circuit is formed are coupled to each other by using a gold wire or gold ribbon 4. Impedance matching is performed through double open stub 2 formed in the orthogonal direction to the series microstrip line 5. In the case that impedance matching is not accurately performed through the open stubs 2, one or both of the two stubs 2 are trimmed in the width or length direction, so that impedance matching can be performed in a target frequency to obtain a maximum return loss.
However, the aforementioned method is used for the lumped type device whose substrate is an n-type substrate and electrode is formed on a metal on a p-type semiconductor formed on the top surface. In the lumped type device, a capacitance and a load resistance are combined, to cause frequency restrictions by an RC time constant. Therefore, the lumped type device cannot be efficiently applied to high frequency such as 40 GHz, 60 GHz
Moreover, since the open stubs 2 for impedance matching are formed in one direction of the microstrip line 5, a spurious mode may occur by interactions between the microstrip line 5 and the open stubs 2.
Devices used for a high speed optical module are classified into a lumped type device and a traveling wave type device. The traveling wave type device can efficiently process 40 GHz and 60 GHz of high speed electric signals. That is, the traveling wave type device prevents frequency restrictions caused by a capacitance with evenly distributing the capacitance. Conversely, a capacitance of the lumped type device is concentrated and coupled to a load resistance, which results in frequency restrictions by the RC time constant. Accordingly, the lumped type device is not appropriate to process high speed signals.
When the impedance is matched for the optical device at a specific frequency, a bandwidth may need to be adjusted so that the signals with different bandwidth which is specified by channel environment, numbers of subscribers, kind of data and so on, can be transferred to subscribers. When the conventional lumped type device is used, an open type termination resistance is applied. Therefore, a bandwidth can be adjusted only by using a few open stubs and the variations are limited.